In general, an acoustic device in the related art, which is represented by a stand-alone type component audio or an acoustic device for vehicle installation, has a reproducing function of a plurality of acoustic sources such as a radio tuner, a CD (Compact Disc), and an AUX (Auxiliary) input that is an external input terminal. Particularly, in the acoustic device for vehicle installation, since a display is provided by fusion with a navigation system, a DVD (Digital Versatile Disc) player and a TV tuner are added as acoustic sources, and the number of acoustic sources has been increased.
In order to adjust a volume with which a user desires to listen to during reproduction of such acoustic sources, the user performs volume adjustment through operating a sound volume. However, in an acoustic device that can select and reproduce any one of a plurality of acoustic sources, if an acoustic source being reproduced is switched to a different sound source, a volume change occurs due to a difference in characteristics (for example, a reproduction band and a kind of an analog/digital signal).
Even if the switching is made in the same acoustic sources, the volume change occurs in a radio receiver or the like due to a difference in modulation degree between radio waves. Further, in the case where a signal is input from an external acoustic device, the volume change occurs depending on the volume of the external acoustic device. To address this problem, in the related art, there is an acoustic device having a volume adjustment function based on the volume balance between acoustic sources beforehand so that no volume change occurs even if the acoustic source is switched.
However, parameters of the volume adjustment are determined at a design stage and have fixed values for respective acoustic sources, and thus it is not possible to change them after shipment. A technology to establish an AGC (Auto Gain Control) circuit in an acoustic device to cope with the above-described case is disclosed, for example, in Japanese Patent Application Publication No. 2001-359184 (Patent Document 1).